1. Field of the Invention
The present invention relates to an air-cleaner filter material for an internal combustion engine, for removing dust etc. from the air, for use in introducing fresh air, which is fit in an air inlet of an internal combustion engine such as an automobile and a general-purpose engine, as well as an air-cleaner element using the same.
2. Description of the Related Art
The air introduced into an internal combustion engine such as in an automobile, a general-purpose engine etc. is clean air made free from dust through an air cleaner, and high efficiency of collection of dust and high capacity for retaining dust are required of the air cleaner. Particularly in recent years, a long-lasting air cleaner having high capacity for retaining dust with less frequency of exchange of the air-cleaner is strongly desired from the viewpoint of reducing waste. To cope with the problem, a density-gradient filter material in a wet or dry system is proposed.
For example, there is disclosed a density-gradient filter material composed of a wet nonwoven fabric having a fiber layer consisting of heat-adhesive fibers and crimped fibers, wherein a resin adhesive is used (see, for example, JP-A No. 4-59007). On one hand, a density-gradient filter composed of a dry nonwoven fabric produced by subjecting fibers containing heat fusion fibers to needle-punching and then thermally treating the fibers is disclosed (see, for example, JP-B No. 53-33787 and JP-A No. 10-180023).
JP-A No. 4-59007 supra proposes a density-gradient filter material in a wet system using heat-adhesive fibers and crimped fibers, but a resin adhesive used in the filter material reduces a space for retaining dust among the fibers, thus shortening the life of an air cleaner using the same. The diameters of the fibers and the weight of the nonwoven fabric are not referred to therein, and the deformation and reduced duration of the filter material attributable to a reduction in strength upon pleating it in an element form are problematic.
JP-B No. 53-33787 and JP-A No. 10-180023 supra disclose respectively a density-gradient nonwoven fabric produced by subjecting fibers containing heat fusion fibers to needle-punching, but needle-punching allows a large amount of minute holes to occur in a dense layer of the fibers, thus causing a problem of re-scattering of dust once captured. There is also a problem that the strength of the filter material is reduced by needle-punching, to permit deformation to occur upon retaining a large amount of dust thereby reducing the capacity for retaining dust.
As described above, a long-lasting filter material sufficiently maintaining the strength of the filter material, suppressing the reduction, by deformation, of the capacity for retaining dust, and having the optimum fiber density gradient is still not obtained at present.